A device shown in FIG. 7 is known generally as such the kind of flow rate measuring device for fluid of the related art (see a patent literature 1, for example). This device is configured by a first ultrasonic transducer 22 and a second ultrasonic transducer 23 disposed in a flow path 21 flowing fluid therethrough; a switching means 24 which switches transmission/reception between the first ultrasonic transducer 22 and the second ultrasonic transducer 23; a transmission means 25 which drives the first ultrasonic transducer 22 and the second ultrasonic transducer 23; an amplifying means 26 which amplifies, with a predetermined amplitude, a received signal received by the ultrasonic transducer on a reception side and passed through the switching means 24; a reference comparison means 27 which compares the voltage of the received signal amplified by the amplifying means 26 with a reference voltage; a determination means 28 which detects from an output signal C that the magnitude relation between the voltage of the received signal and the reference voltage is reversed as a result of the comparison therebetween by the reference comparison means 27 as shown in FIG. 8 and outputs an output signal D to a repetition means 29 at a timing of a first zero cross point a of the amplified signal generated thereafter; the repetition means 29 which counts the output signal D from the determination means 28 and outputs the signal from the determination means 28 to a control means 32 when the count number reaches a preset number; a clock means 30 which measures a time required for the repetition means 29 to count the preset number; a flow rate calculation means 31 which calculates a flow rate in accordance with the time measured by the clock means 30; and the control means 32 which receives an output representing the calculated flow rate from the flow rate calculation means 31 and the signal from the repetition means 29 to thereby control the operation of the transmission means 25.
According to this device, when the control means 32 operates the transmission means 25, an ultrasonic signal generated from the first ultrasonic transducer 22 propagates within the flowing fluid and is received by the second ultrasonic transducer 23. Then, the received signal is amplified by the amplifying means 26, then subjected to the signal processings by the reference comparison means 27 and the determination means 28, and inputted into the clock means 30 and the control means 32 via the repetition means 29. Then, the switching means 24 performs the switching between the first ultrasonic transducer 22 and the second ultrasonic transducer 23, and an operation similar to the aforesaid operation is performed. This operation is repeatedly performed for a preset number of times n, whereby the clock means 30 measures a propagation time of the fluid to be measured to the downstream from the upstream (flow in this direction is called a forward flow) and a propagation time thereof to the upstream from the downstream (flow in this direction is called a reverse flow). Then, the flow speed of the fluid to be measured is obtained and a flow rate Q is obtained by the following (expression 1).
Supposing that an effective distance between the ultrasonic transducers in the flowing direction is L, a measured time of n-times to the downstream from the upstream is t1, a measured time of n-times to the upstream from the downstream is t2, the flow speed of the measured fluid is v, the sectional area of the flow path is S, a sensor angle is φ and the flow rate is Q, the following expression is obtained.Q=S·v=S·L/2·cos φ((n/t1)−(n/t2))  (expression 1)
In fact, the flow rate is obtained by further multiplying the expression 1 by a coefficient according to the flow rate.
Further, the gain of the amplifying means 26 is adjusted so that the amplitude of the signal received by the ultrasonic transducer on the reception side becomes constant in a manner that the maximum voltage of the received signal is rendered within a predetermined voltage range. In this adjustment, during the measurements repeatedly performed for the number of times set by the repetition means 29, the number of times that the maximum voltage of the received signal becomes lower than the lower limit of the predetermined voltage range like a received signal b shown by a dotted line in FIG. 9 is counted, and also the number of times that the maximum voltage becomes higher than the upper limit of the predetermined voltage range like a received signal c shown by a dotted line in FIG. 9 is counted. Then, the gain at the time of the next flow-rate measurement is adjusted in accordance with the magnitude relationship between these counted numbers. For example, when the number of times that the maximum voltage becomes lower than the lower limit is larger, the gain is increased to thereby set the maximum voltage so as to be between the upper limit and the lower limit of the voltage range like a received signal a shown by a steady line in FIG. 9.
The reference voltage of the reference comparison means 27 to be compared with the received signal amplified by the amplifying means 26 is used to determine the position of a zero cross point detected by the determination means 28. As shown in FIG. 8 as an example, the reference voltage is set to the center point between the peak voltage of the third waveform and the peak voltage of the fourth waveform of the received signal so that the determination means 28 detects a zero cross point a of the fourth waveform of the received signal. Thus, even when the peak voltage of the third waveform of the received signal increases or the peak voltage of the fourth waveform reduces due to some reason, since the reference voltage has some margin with respect to each of these peak voltages, the determination means 28 can stably detect the zero cross point a of the fourth waveform.